The present invention relates to a wire bonding method which comprises electrically connecting pads of a semiconductor chip bonded to a substrate and electrodes on the substrate using an electrically-conductive wire and particularly, to a wire bonding method by which a bonding can be secured even when the thickness of a plated gold film of the electrodes provided on the substrate is thin.
For example, the wire bonding method has been widely employed which comprises forming electrodes on a substrate made of a glass epoxy as a base material, mounting semiconductor chips on the substrate and connecting the pads of the semiconductor chip with the electrodes through wires comprising a superfine gold wire.
The electrodes are formed by applying gold plating onto a copper base formed on the substrate. However, the gold plating is seldom applied directly onto the copper base, and in many cases, cheap nickel having a proper hardness is plated on the copper base for inhibition of the surface stain caused by diffusion of copper atoms. That is, the electrode has a three-layer structure consisting of a copper layer, a nickel layer and a gold film on the substrate.
It is necessary that after the electrode has been bonded to the wire, a sufficient bond strength is kept at the portion where the lower end of the wire and the electrode have been bonded. Hitherto, it has been experientially considered that the bond strength varies depending on the thickness of the gold film and the thickness of the gold film must be increased (for example, to more than 0.5 .mu.m) in order to obtain a sufficient bond strength, but no exact cause therefor has been known. Therefore, practically the wire bonding has been carried out after forming a thick gold film to secure the sufficient bond strength.
However, there is the problem that the costs unavoidably increase when the wire bonding is effected with forming a thick gold film.
The present inventor has found the causes for occurrence of reduction in the bond strength.